Since the turn of the millennium, cannabis reform has swept the world. Countries spanning five continents have ended the blanket prohibition of cannabis, boldly opting for a range of alternatives, from decriminalisation, medical use, heavily state-controlled sales or unfettered capitalism.
With two and a half decades of data available, a comprehensive review published in The Lancet Psychiatry has examined the effects of reform on usage rates, links to poor mental health outcomes and wider impacts on society. The results once again challenge the narrative that prohibition is the best route to keeping the public safe, while warning that the way countries legalise cannabis matters the most.
25 years of cannabis reform
In the review, researchers at the University of Bath analysed cannabis policy changes across multiple countries in the Americas, Europe, Africa, Asia and Oceania. The team examined the links between reform and changes in cannabis use, cannabis use disorder, and psychiatric outcomes including psychosis, anxiety and depression.
Drawing on population surveys, health records and longitudinal studies in leading health publications from January 1, 2000 to March 18, 2025, the review is the biggest international assessment of cannabis reform ever undertaken, with each model showing a clear pattern: how cannabis is regulated matters far more than whether it is illegal or not. Decriminalisation, medical access and close state regulation show little to no increase in public harm, but turning the plant over to profit-driven markets tells a different story.
Two decades of European decriminalisation
Portugal led the world in harm reduction in 2001, decriminalising the possession of all drugs, and treating substance use as a health issue rather than a criminal one.
The data from this radical experiment have been overwhelmingly positive across all substances, including cannabis. In the period following decriminalisation (2001-07), past-year cannabis use in Portugal increased by just 0.3% – slightly less than the 0.4% increase seen in neighbouring Spain, and far less than Italy’s 8.4% rise in the same period.
This pattern has repeated across other European countries that have followed suit. When Czechia decriminalised cannabis possession in 2010, the Lancet review found no significant association between the policy change and the age at which people first tried cannabis. When the UK downgraded cannabis from Class B to Class C between 2004 and 2009, the review found no substantial changes in use amongst adolescents aged 15-17 or young adults aged 18-20. Other drug use and crime indicators remained stable. Notably, hospital admissions for cannabis-induced psychosis actually declined during this period.
The review found these patterns were consistent with broader decriminalisation policies. One study examining cannabis policy changes across 38 countries between 2001 and 2010 found no consistent significant associations between liberalisation and cannabis use among adolescents. “The liberalisation of cannabis policies in Belgium, France, Finland, and Portugal between 1999 and 2004 was not associated with changes in the demand for treatment for cannabis use disorder,” the authors said.
Beyond Europe’s borders
These results were consistent with other decriminalisation experiments across the world. After South Africa’s Constitutional Court declared it unconstitutional to criminalise adults’ private use and cultivation of cannabis in 2018, data showed no immediate increase in cannabis related hospital admissions amongst adolescents between 2015 and 2020. When Georgia decriminalised the possession of small amounts of cannabis in 2018, the annual prevalence of cannabis use among young adults aged 18-29 remained essentially unchanged.
Lessons from Uruguay
Uruguay became the first to adopt a fully legal and regulated cannabis policy at national level, but instead of turning control over to the free market, the government allowed access through three channels: home growing of up to six plants, non-profit cannabis social clubs and state-licensed pharmacies.
Commercially, cannabis is tightly controlled. No advertising, no branded products and caps on THC levels.
When comparing data from the periods before and after legalisation, the review showed that cannabis use among people below the legal age declined across every measure. Frequent use (defined as 10 or more days per month) dropped by 1.8 percentage points, while high-risk use fell by 5.0 percentage points. Among those aged 18-21 who could legally access cannabis, high-risk use decreased by 3.5 percentage points. Rather than triggering an epidemic of youth cannabis use, Uruguay’s tightly regulated model achieved the opposite: it reduced harm among the population prohibitionists claimed to be protecting.
Where reform went wrong
While other countries have led with public health-focused policies, North America has broadly adopted ‘for-profit’ models. It’s here the review found mixed results.
Canada legalised cannabis nationally in 2018. Unlike Uruguay, provinces largely adopted a commercial model with private retail stores with varying degrees of regulation. Commercialisation has driven consumption. While some consequences have been positive, such as a reduction in youth use and fewer alcohol and opioid related deaths, adult cannabis use has increased from 22% in 2018 to 27% by 2020, stabilising at 26% by 2023-24.
In the US, a patchwork of state-level reforms reveals varying outcomes, but the review is clear in its findings that “evidence from Canada and the USA indicates that legalisation of non-medical cannabis (with commercial supply) has been associated with increases in the prevalence of cannabis use and cannabis use disorder among adults.”
The researchers found evidence that the need to advertise and sell stronger, cheaper cannabis based products to achieve commercial objectives was linked to an increased risk of cannabis use disorder and other psychiatric conditions. Some data showed that emergency department visits related to psychosis increased in some jurisdictions – particularly in Colorado where visits rose as the ratio of dispensaries to population increased. But one larger multi-state analysis in the review found no significant association between cannabis policies and rates of psychosis-related health outcomes.
“The Lancet paper demonstrates what has long been obvious to drug policy analysts: commercial incentives to maximise profits generally means increasing sales and consumption, which inevitably drives product development and corporate marketing towards sub-optimal public health outcomes,” Steve Rolles, senior policy analyst at Transform Drug Policy Foundation, told leafie.
Commercially driven cannabis markets were linked to higher rates of cannabis use disorderWhat distinguishes the results from North America when compared to Uruguay or European decriminalisation is the scale and nature of commercialisation. The researchers identified this as the critical factor in understanding why some reforms showed negative outcomes, and others didn’t. The biggest threat to the wider public isn’t cannabis itself, but how reform rolls out and who profits. Where cannabis is treated like another vice, like drinking and smoking, problems are more likely to occur.
“This has been a core theme of historic efforts to better regulate alcohol and tobacco markets. It highlights the very real tensions between commercial profit incentives to increase use and public health priorities to moderate it”, Rolles warned.
“The penetration of big tobacco and alcohol into the cannabis sector, alongside market consolidation of the industry in North America and increasingly in Europe and globally, should be viewed as a profound threat to responsible public-health-led policy making in this field.”
Cannabis reform is still better than prohibition
One key finding from the review is that even in commercialised markets, the harms of reform done badly need to be weighed against the harms of continued criminalisation. Keeping cannabis in the hands of criminals provides the worst outcome of all.
Here in the UK, a 2025 report by the London Drugs Commission found that Black people are five times more likely to be stopped and searched on suspicion of drug-related offences than white people. Cannabis possession accounts for the majority of these stops, creating a system where prohibition’s costs fall disproportionately on racialised communities. A 2018 study found that over a million hours of police time a year are wasted enforcing cannabis bans, and that criminalisation costs the public over £890 million annually in England and Wales.
The criminal justice costs are only part of the picture. Criminal convictions create barriers to employment, housing and education that can last decades. The illegal market generates violence that regulated markets – even imperfect commercial ones – can reduce. What is clear from the review is that criminalisation simply isn’t working for anyone.
What the UK should learn
Even at its most critical, the review’s findings don’t create a strong case for prohibition. They show that reform matters – and that it works – but policy design choices have consequences; how we reform matters as much as whether we reform.
For policymakers that prioritise public health, this analysis lays an evidence-backed path forward. “Non-commercial models, including regulated home-growing, and not-for-profit membership-based associations like those we have seen established in Uruguay, Malta and Germany” offer the strongest evidence base for successful reform, according to Rolles. “Where more conventional retail is permitted, developing strictly regulated market models that control product form and potency, branding and marketing is essential.”
This doesn’t mean that legalisation should avoid all commercial activity, but rather acknowledging cannabis as a drug with a risk profile that justifies stricter controls than conventional products.
“The Lancet paper essentially reinforces the message that we need to use the opportunities offered by legalisation to do things differently and better,” Rolles argues. “To design new market infrastructure that deprioritises or eliminates corporate profit incentives, and that primarily serves the interests of public health and community safety.”
With over 25 years of data available, the argument for criminalisation can no longer be made, agreed Ian Hamilton, an Associate Professor in addiction at the University of York. “This review gathers a significant amount of data over many years, providing the most comprehensive view of the impact various policies on access to cannabis have on populations.”
“This evidence needs to be examined by the UK government and should prompt a review of the current legislation, which has no basis in evidence,” Hamilton added. ”There is no evidence of increased harm to mental health when jurisdictions allow access to cannabis for individuals.”
What is clear is that regulated access to cannabis, whether via commercial organisations or governments, does not end in a public health disaster, as many have forecast. Cannabis reform doesn’t have to mean cannabis harm. 25 years of data show that it is possible to implement reforms that prioritise public health before profit, that new laws can successfully regulate safe adult use and end the injustices of prohibition. The challenge now is to make policymakers listen.
